Robinson — On the Cup Anemometer. 439 



cular -weights, up to 10 or 12 lbs., can be placed; below which is 

 attached a conical vessel to receive shot for small additions of weight, 

 which can be removed by a valve at its bottom. 



It is important that the machine shall be capable of being driven 

 with a uniform speed of any amount, which can be gradually increased. 

 This may best be effected by the descent of a weight connected with 

 the axle, and continually wound up by means of an arrangement like 

 the well-known contrivance of Huyghens. The velocity can be increased 

 by adding to the weight. At the speed of 25 miles per hour, the 

 maximum resistance to the cups would be about 2 lbs., and the power 

 expended that of 75 lbs., falling 1 foot in a second. This is only f of 

 a man's power. 



The Anemometer to be used in connexion with this machine should 

 also present as little resistance to the air as possible. For this pur- 

 pose its frame should consist of a strip of sheet iron, twice bent at right 

 angles. The uprights so formed should have bearings for the axle, 

 which mtist be set parallel to the arm of the whirling machine, and 

 need not be more than 6 inches long and ^ inch in diameter. The 

 cross of the Anemometer is fixed to its outer extremity : this should, 

 in the first instance, be of the Kew type, cups of 9 inches, and the 

 track of their centres 48 diameter. On the axle is secured a brass disk, 

 6 inches diameter, on whose circumference acts a circular brake, one 

 lug of which is screwed firmly to the bottom of the frame, the other 

 one is pressed towards it by the short arm of a right-angled lever, 

 turning on a centre similarly secured, and its longer arm connected 

 with the cord coming from the whirling machine.* It is evident that 

 by placing weights on the stage, we can apply considerable pressure 

 to the brake, and thus increase the Anemometer's friction without at 

 all interfering with the whirl. Any of these frictions is easily 

 measured. Let the mouth of a cup be horizontal ; place small weights 

 at its centre till it just moves on slightly, jarring the frame as by 

 light taps, or cbawing a float over it. Repeat this for the other cups, 

 and take the mean. This force is less than what is required to start 

 them from a state of rest, but it corresponds to that which will exist 

 during the rotation of the machine. I expect it will be found thatthe 

 friction will be constant for a given load on the stage. The centrifugal 

 friction may be determined by attaching to the outer end of the Anemo- 

 meter axle a thread, pulling in its direction over a pulley of known 

 friction, apply to it a weight = P. and measure the friction ; the excess 

 of this over the normal friction divided by P = fi,,. As it is desirable 

 that the normal friction should be kept as low as possible, the axle 

 should rest on vertical friction wheels, and a horizontal one bearing 

 on the back of the brake disk will lessen the centrifugal one. For a 



* Mr. Grubb suggests that tiie brake apparatus sbould be broiight close to the 

 ^vertical axis, and the axle of the Anemometer lengthened This woiild materially 

 'lessen the disturbance of air caused by the whirl, and the centrifugal friction. 



